That it is, and I couldn't help but notice that it retroactively feels like a reference to Yoda's and Sidious' battle in Revenge of the Sith.

The "if-it-didn't-happen-on-screen-it-didn't-happen" rule corresponds only to the films themselves (which, as I mentioned, has the unfortunate unspoken addition of "It also happened if George Lucas said so"). This isn't the same as video games. Yes, the game has been declared canon, but it wasn't the first time; almost all Star Wars games are canon. C-Canon, that is. The films are G-Canon. Even if George Lucas pointedly declares that he considers the events of a certain game, novel or comic book to have "truly occurred" within his universe, it doesn't automatically elevate that piece of media to G-Canon (because he's also made the same declaration about Knights of the Old Republic as well as the Dark Empire comic series, the latter of which he would later come to denounce).

So just because something has George Lucas' stamp of approval doesn't automatically put it on the same level of canonicity as the movies. After all, Lucas has the notorious habit of going back on his word and contradicting those stories, or even flat-out disowning them.

Further, how can The Force Unleashed possibly be G-Canon if Lucas' own animated television series, The Clone Wars, isn't even that high? (It's on it's own level: "T-Canon," which a new category and the second highest, between G and C.)

Yes, The Force Unleashed is canon, but in the same sense that the rest of the Expanded Universe is canon.

As for your points about Starkiller, they are fairly convincing, except for one small factor: the Dark Apprentice from the dark side ending and the "Distant Thunder" videos. Now, there's a perfect clone of a Force user, as Vader stated. They can't both be the real Starkiller. (And let's not get into the whole debate over whether or not the Dark Apprentice truly exists despite the non-canon ending. I've made this argument at length in the past. In short, the only logical conclusion is that he does exist.)